1. Field of the Invention
The present invention relates to projection tubes of the type including a target having a fluorescent screen adapted to be irradiated with an electron beam and means for projecting in front the image produced on the fluorescent screen, and more particularly the invention relates to an improved projection tube including an X-ray shield disposed to cover the surfaces of the target excluding the fluorescent screen.
2. Description of the Prior Art
A known type of projection tube will now be described with reference to FIGS. 1, 2, 3, 4 and 5.
Generally, with the projection tube, as shown in FIG. 1, an electron beam 14 modulated by a picture signal and emitted from an electron beam 2 is focussed, deflected and then projected onto a fluorescent screen 3 on a target 4 which is carried on a supporting ring 6 by supporting means. The light emitted from the fluorescent screen 3 is reflected forward by a concave mirror 5 fixed to the other end of the supporting ring 6, and the reflected light reproduces an image on a screen in front through a faceplate 7 and a correction lens 8.
The target 4 and its supporting structure are shown in FIGS. 2A and 2B. Four supporting arms 10 each has its one end welded to a supporting plate 9 and its other end fixed by welding to the inner surface of the supporting ring 6. After the fixing by welding, the target 4 is fixed to the supporting plate 9 by screws 11 thus completing the assembly. Since the target 4 must be formed with the fluorescent screen 3 on one surface and since the target 4 itself tends to increase in temperature, the target 4 is made of aluminum material having excellent heat dissipating properties and ensuring easy formation of a fluorescent screen. When the target 4 made of such material is irradiated with the electron beam 14, X-rays are produced whose intensity is strong enough to pass through the thickness of the target 4. When the electron beam 14 is struck to the positions of points A and B on the target 4 as shown in FIG. 3, the resulting X-rays are transmitted through the interior of the target 4 and the X-rays are thus radiated to the front of the projection tube as shown by dotted lines 17 through its surface where the supporting plate 9 made of iron or the like is not present. The amount of the X-rays transmitted to the front was measured at position C, D and E in FIG. 4 with an X-ray measuring instrument (Victreen 440 RF/C) having an X-ray detecting means of 5 cm.sup.2 and the results shown in FIG. 5 were obtained. It should be noted that some of the measurements were in excess of the value set by the HEW or 0.5 mR/h.
To overcome this problem of the X-ray transmission to the front of the projection tube, methods heretofore proposed are to use for the faceplate 7 and the correction lens 8 a glass material containing a large amount of lead or to increase the distance of the correction lens 8 from the faceplate 7 to such an extent which is sufficient to cause attenuation of X-rays. However, these proposed methods, particularly the latter is disadvantageous structurally in that the correction lens 8 must be made integral with the projection tube proper by molding or the like and consequently this has the effect of increasing the size of the apparatus using the tube and also making the positioning of the correction lens 8 impossible.